The present invention relates to fan blades for gas turbine engines and, more particularly, to an apparatus and method for maintaining a fan blade in a desired radial orientation and providing added protection to the structural integrity of the root region of a fan blade when the fan blade collides with a foreign object.
In the past, aircraft gas turbine engine composite fan blades with cylindrical pressure faces have not been designed to stay in a fixed radial orientation. A cylindrical-pressure-face connection of a fan blade with a rotor disk is very similar to a ball-joint connection. Typically, the root and dovetail regions of the fan blade fit into an accommodating cavity of a rotor disk and are given such a degree of freedom that their radial orientation is not constantly maintained. The radial orientation of such fan blades differs depending on whether the engine is idling, windmilling, or under full power. Once rotation of the fan blades begins, friction in the dovetail regions of the fan blades tends to seat the blades at a position that the blade stacking axis may be off center or misaligned from a desired blade radial position. It is desirable that the blade stacking axis remain a constant from blade to blade so that fan rotor balance does not change with each operation for purposes of preventing unacceptable vibrations.
Another problem encountered by fan blades in aircraft gas turbine engines is the ingestion of foreign objects into the air inlet at the front of the nacelle and collision of such objects with the fan blades. These foreign objects often manifest themselves as birds or runway debris which are sucked into the fan blades. It is desirable that the damage to a fan blade struck by a foreign object be confined to the immediate area of impact, typically the airfoil region of the fan blade. If impact with a foreign object causes the root region of the fan blade to be separated from its connection to the rotor disk, serious damage can be done to the engine.
In an exemplary engine, each fan blade may only weigh approximately 30 pounds. However, when the blades are spinning, each blade can have as much as 200,000 pounds of load applied to the disk-root interface. Therefore, the importance of keeping the root region of the blade intact with the rotor disk can be clearly appreciated.
Thus, a need exists for a fan blade and disk assembly which will allow a fan blade to maintain a desired radial position during start up and running. Also, it is desirable to reduce the bending moment at the root of the fan blade when the fan blade is struck by a foreign object.